Abstract

Deformability is often a crucial to the conception of many civil-engineering structural elements.
Also, design is all the more burdensome if both long- and short-term deformability has to be
considered. In this thesis, long- and short-term deformability has been studied from the material
and the structural modelling point of view. Moreover, two materials have been handled: pultruded
composites and concrete.
A new finite element model for thin-walled beams has been introduced. As a main assumption,
cross-sections rigid are considered rigid in their plane; this hypothesis replaces that of the classical
beam theory of plane cross-sections in the deformed state. That also allows reducing the total
number of degrees of freedom, and therefore making analysis faster compared with twodimensional
finite elements. Longitudinal direction warping is left free, allowing describing
phenomena such as the shear lag. The new finite-element model has been first applied to concrete
thin-walled beams (such as roof high span girders or bridge girders) subject to instantaneous service
loadings. Concrete in his cracked state has been considered through a smeared crack model for
beams under bending.
At a second stage, the FE-model has been extended to the viscoelastic field and applied to pultruded
composite beams under sustained loadings. The generalized Maxwell model has been adopted.
As far as materials are concerned, long-term creep tests have been carried out on pultruded
specimens. Both tension and shear tests have been executed. Some specimen has been strengthened
with carbon fibre plies to reduce short- and long- term deformability. Tests have been done in a
climate room and specimens kept 2 years under constant load in time.
As for concrete, a model for tertiary creep has been proposed. The basic idea is to couple the
UMLV linear creep model with a damage model in order to describe nonlinearity. An effective
strain tensor, weighting the total and the elasto-damaged strain tensors, controls damage evolution
through the damage loading function. Creep strains are related to the effective stresses (defined by
damage models) and so associated to the intact material.

Abstract

Deformability is often a crucial to the conception of many civil-engineering structural elements.
Also, design is all the more burdensome if both long- and short-term deformability has to be
considered. In this thesis, long- and short-term deformability has been studied from the material
and the structural modelling point of view. Moreover, two materials have been handled: pultruded
composites and concrete.
A new finite element model for thin-walled beams has been introduced. As a main assumption,
cross-sections rigid are considered rigid in their plane; this hypothesis replaces that of the classical
beam theory of plane cross-sections in the deformed state. That also allows reducing the total
number of degrees of freedom, and therefore making analysis faster compared with twodimensional
finite elements. Longitudinal direction warping is left free, allowing describing
phenomena such as the shear lag. The new finite-element model has been first applied to concrete
thin-walled beams (such as roof high span girders or bridge girders) subject to instantaneous service
loadings. Concrete in his cracked state has been considered through a smeared crack model for
beams under bending.
At a second stage, the FE-model has been extended to the viscoelastic field and applied to pultruded
composite beams under sustained loadings. The generalized Maxwell model has been adopted.
As far as materials are concerned, long-term creep tests have been carried out on pultruded
specimens. Both tension and shear tests have been executed. Some specimen has been strengthened
with carbon fibre plies to reduce short- and long- term deformability. Tests have been done in a
climate room and specimens kept 2 years under constant load in time.
As for concrete, a model for tertiary creep has been proposed. The basic idea is to couple the
UMLV linear creep model with a damage model in order to describe nonlinearity. An effective
strain tensor, weighting the total and the elasto-damaged strain tensors, controls damage evolution
through the damage loading function. Creep strains are related to the effective stresses (defined by
damage models) and so associated to the intact material.